Here is an article on Mario Super Sluggers from some random Japanese Nintendo Magazine.
-9 Total Stadiums to play in (Possible 10th one is unlocked?)
-More extensive Story Mode.
-Up to 4 players can play all Modes.
-Every character has special pitching & batting abilities: Mario throws / hits a massive Fireball, Luigi uses the Tornado Ball, Yoshi has an Egg Rainbow move & Bowser Jr. sends splatters of paint everywhere which hide where the ball is.
-New Defensive Items can be used such as a Cannon that shoots a Bob-Omb at a fielder who is just about to catch your pop-up fly ball. This is done by pointing at the screen with the Wii Remote and pressing the B trigger to fire.
-Play during the Day or at Night in Stadiums.
-Play as your Miis.
-Perform Special character moves in the outfield. For example: Jump on Mario’s head for an extra boost into the air to catch fly balls.
-The game is running on a brand new engine, built from the ground up for Wii. Not a rehashed engine of the Gamecube game. This means higher res textures, more detail in animations, nicer effects, smother looking characters, etc.
-Shake Wii Remote to run faster, but not necessary.
-Swing Wii Remote to hit the ball when Batting.Your results will vary based on the strength and timing of your movement.
-Swing Wii remote to pitch. You can add spin and curve to the ball if your twist your wrist when pitching. You can also pitch faster by swinging harder.